The concept of monitoring, regulating, and maintaining tire pressure on a moving vehicle is not new. Tire pressure maintenance has always been important and the transportation industry has constantly promoted this subject as a means to increase tire life and fuel economy, while reducing down time and maintenance costs due to flat tires. A tire blow out on the road can leave debris that can be hazardous to other drivers. Tire pressure maintenance can decrease the possibility of tire failures and increase safe operation for the vehicle driver and other drivers on the road.
Various systems have been designed and are the subject of numerous patents that accomplish the objective of tire pressure maintenance. These systems are typically called central tire inflation systems (CTIS). The most common systems in the heavy truck industry are designed for trailers. Trailer axles are typically hollow with axle ends that commonly have a through bore. The hollow axle provides an advantageous conduit to supply air pressure to the wheel end. Steer axles and driven axles are more challenging due to typically solid spindles on steer axles and solid rotating shafts inside drive axles.
In all cases (steer, driven, and trailer) the wheel end assembly includes a lubrication area between the axle and the wheel end further defined by plugging the through bore in the axle end and covering the end of the axle with a hub cap attached to the wheel end. The wheel end is supported on the axle end by wheel bearings. The bearings require lubrication and the integrity of the lubrication area is essential in maintaining the operability and life of the wheel end assembly.
In order to provide pressurized air to the rotating tires, the CTIS typically includes a rotary union in the same general location as the bearings and necessarily in the lubrication area between the stationary axle and the rotating wheel end. The most critical function of the CTIS is to not allow pressurized air into the lubrication area. The lubricating oil can be forced past the wheel end seals by air pressure, leading to bearing failure and consequently catastrophic wheel end failure.
Two methods are commonly used in prior art to supply pressurized air to the end of a trailer axle. The first is to pressurize the axle itself. The second is the use of a smaller conduit, such as an air hose, inserted within the axle. U.S. Pat. No. 3,276,503 to Kilmarx (Kilmarx U.S. Pat. No. 3,276,503) teaches the use of plastic tubes inside a hollow axle to supply air to the axle end. U.S. Pat. No. 839,626 to Nielsen, (Nielsen U.S. Pat. No. 836,626) discloses the use of a conduit in the axle itself to conduct the air to the axle end using a fitting in the axle to supply air to the conduit. In either case the air pressure at the end of the axle is then communicated from the stationary axle to the rotating wheel by the use of a rotary union.
Many configurations of rotary unions have been disclosed. Kilmarx U.S. Pat. No. 3,276,503 discloses a rotary air union as part of the hub cap, rotating with the wheel. U.S. Pat. No. 5,584,949 to Ingram (Ingram U.S. Pat. No. 5,589,949) locates the rotary air union in the axle end. U.S. Pat. No. 6,425,427 to Stech (Stech U.S. Pat. No. 6,425,427) splits the rotary union between the rotating hub cap and the stationary axle end. In Kilmarx U.S. Pat. No. 3,276,503, Ingram U.S. Pat. No. 5,589,949, as well as U.S. Pat. No. 6,105,645 to Ingram et al. (Ingram II U.S. Pat. No. 6,105,645) and U.S. Pat. No. 6,145,559 to Ingram (Ingram III U.S. Pat. No. 6,145,559), the rotary union is self-contained where a stationary shaft is bearing mounted to a rotating housing with a seal element. In Stech U.S. Pat. No. 6,425,427, the rotary union is made up of multiple parts that interface during assembly, and in U.S. Pat. No. 5,769,979 to Naedler, the stationary seal and rotary shaft are separate until assembly. In any case the primary function of the rotary air union is to deliver pressurized air from the stationary source to the rotating tire.
The primary seal within the rotary air union also takes various forms. Nielsen U.S. Pat. No. 839,626 uses a spring loaded face seal arrangement, Kilmarx U.S. Pat. No. 3,276,503 also uses a spring loaded seal element biased against an abutting interface, and in U.S. Pat. Nos. 1,016,896, 1,112,596, 2,156,841, and 2,715,430, an o-ring or packing material seals on the outside diameter of the shaft of the rotary union assembly. In each case the primary seal disclosed is the single seal that exists to maintain fluid tight integrity within the rotary union.
The primary seal is a key element of the rotary union. However, in even the best conditions the primary seal can fail and pressurize the lubrication area. To address this issue a number of patents include a vent to atmosphere. Ingram II U.S. Pat. No. 6,105,645 includes a vent in the rotary housing proximal to the primary seal, Ingram III U.S. Pat. No. 6,145,559 provides a vent in the bulkhead assembly attached to the axle, and Ingram U.S. Pat. No. 5,584,949 includes a vent in the hub cap. In all three cases the vent location is inside the lubrication area allowing air pressure to escape to atmosphere.
U.S. Pat. No. 6,325,124 to Colussi discloses a secondary seal and a vent to atmosphere, outside the lubrication area, between the primary seal and the secondary seal. The secondary seal is not in contact with the stem until air pressure is present. There is a vent hole between the two seals in order to vent air escaping the primary seal. The disclosure of this feature in Colussi U.S. Pat. No. 6,325,124 is to provide a warning device for wheel bearing failure. If the wheel bearings fail, the stem will retract from the primary seal allowing tire inflation system air pressure to escape. Air pressure escaping the primary seal will exit the vent hole between the two seals and warn the operator of a bearing problem. The vent is a direct, open conduit from atmosphere to the space between the seals.
A further objective of the rotary union is to supply air from the stationary axle to the rotating wheel and tire when misalignment between the two is present. Varied approaches to this challenge have been disclosed. Kilmarx U.S. Pat. No. 3,276,503 uses a flexible conduit element between the axle end and the rotary union, Ingram III U.S. Pat. No. 6,145,559 uses a flexible conduit between the axle and rotary union, while Ingram II U.S. Pat. No. 6,105,645 uses a flexible hose. In each case the flexible element and its associated seals and connections are torque carrying elements between or within the stationary and rotating components of the rotary union.
Kilmarx U.S. Pat. No. 3,276,503 drives torque through an o-ring seal, a flexible conduit, and a threaded connection, Ingram III U.S. Pat. No. 6,145,559 drives torque through a flexible conduit and a connector, and Ingram U.S. Pat. No. 5,584,949 drives torque through a flexible hose, an o-ring, and a fitting.
In regard to the prior art, improvements can be made to the rotary union assembly of a central tire inflation system. There is a need in the art for a seal arrangement that provides a positive back-up seal as a barrier to system air entering the lubrication area. There is also a need in the art for a vent to atmosphere be located to relieve CTIS air pressure while not pressurizing the lubrication area and not allowing contaminates to enter the seal area. Finally, there is a need in the art for an element that allows for misalignment between the stationary and rotary components of the rotary air union, while isolating the stationary components from excessive torque.